This is the old Howard Frankland Bridge that
carries roughly 180,000 vehicles per day across Old Tampa Bay between St. Petersburg and Tampa,
Florida. A replacement for the bridge is currently
under construction, but the Florida Department of Transportation almost had to replace it
decades earlier. The bridge first opened for traffic in 1960,
but by the mid-1980s it was already experiencing severe corrosion to the steel reinforcement
within the concrete members. After less than 30 years of service, FDOT
was preparing to replace the bridge, an extremely expensive and disruptive endeavor. But, before embarking on a replacement project,
they decided to spend a little bit of money on a test, a provisional retrofit to try and
slow down the corrosion of steel reinforcement within the bridge’s substructure. Over the next two decades, FDOT embarked on
around 15 separate corrosion protection projects on the bridge. And it worked! The Howard Frankland Bridge lasted more than
60 years in the harsh coastal environment before needing to be replaced, kept in working
condition for a tiny fraction of the cost of replacing it in the 1980s. The way that bridge in Tampa was protected
involves a curiously simple technique, and I’ve built a ridiculous machine in my garage
so we can have a corrosion protection shootout and see how it measures up. I’m Grady and this is Practical Engineering. In today’s episode, we’re talking about
cathodic protection for corrosion control. This video is sponsored by Curiosity Stream
and Nebula. More on them later. Of all the structural metals in use today,
most applications in infrastructure consist of mild steel (just plain old iron and carbon). There are so many applications where steel
infrastructure comes into contact with moisture, including bridges, spillway gates, water tanks,
and underground pipelines. That means there are so many opportunities
for rust to deteriorate the constructed environment. We’re in the middle of a deep dive series
on rust, and in the previous video about corrosion, I talked about its astronomical cost, which
equates to roughly $1,400 per person per year, just in the United States alone. Of course, we could build everything out of
stainless steel, but it’s about 5 times as expensive for the raw materials, and much
more difficult to weld and fabricate than mild steel. Instead, it’s usually more cost effective
to protect that mild steel against corrosion, and there are a number of ways to do it. Paint is an excellent way to create a barrier
so that moisture can’t reach the metal, and I’ll cover coatings in a future video. But, there are some limitations to paint,
including that it’s susceptible to damage and it’s not always possible to apply (like
for rebar inside concrete). That’s where cathodic protection comes in
handy. Let me introduce you to what I am calling
the Rustomatic 3000, a machine you’re unlikely to ever need or want. It consists of a tank full of salt water,
and a shaft on a geared servo. These plastic arms lower steel samples down
into the saline water and then lift them back up so the fan can dry them off, hopefully
creating some rust in the process. Corrosion is an electrochemical process. That just means that it’s a chemical reaction
that works like an electrical circuit. The two individual steps required for corrosion
(called reduction and oxidation) happen at separate locations. This is possible because electrons can flow
through the conductive metal from areas of low electric potential (called anodes) to
those of high potential (called cathodes). As the anode loses electrons, it corrodes. This reaction is even possible on the same
piece of metal because different parts of the material may have slightly different charges
that drive the corrosion cell. However, you can create a much larger difference
in electric potential by combining different metals. This table is called the galvanic series,
and it shows the relative inertness or nobility (in other words, resistance to corrosion)
of a wide variety of metals. When any two of these materials are joined
together and immersed in an electrolyte, the metal with lesser nobility will act as the
anode and undergo corrosion. The more noble metal becomes the cathode and
is protected from corrosion. You can see that steel sits near the bottom
of the galvanic table, meaning it is less noble and more prone to corrosion. But, there are a few metals below it, including
some commonly available ones like Aluminum, Zinc, and Magnesium. And wouldn’t you know it, I have some pieces
of Aluminum, Zinc, and Magnesium here in my garage that I attached to samples of mild
steel in this demo. We can test out the effects of cathodic protection
in the rustomatic 3000. Each time the samples are lifted to dry, the
arduino controlling the whole operation triggers a couple of cameras to take a photo. One of the samples is a control with no anode,
then the other three have anodes attached consisting of magnesium, aluminum, and zinc
from left to right. I’ll set this going and come back to it
in a few minutes your time, three weeks my time. One application of cathodic protection you
might be familiar with is galvanizing, which involves coating steel in a protective layer
of zinc. The coating acts kind of like a paint to physically
separate the steel from moisture, but it also acts as a sacrificial anode because it is
electrically coupled to the metal. Galvanizing steel is relatively inexpensive
and extremely effective at protecting against corrosion, so nearly all steel structures
exposed to the environment have some kind of zinc coating, including framing for buildings,
handrails, stairs, cables, sign support structures, and more. Most outdoor-rated nails and screws are galvanized. You can even get galvanized rebar for concrete
structures, and there are applications where it is worth the premium to extend the lifespan
of the project. But because it’s normally a factory process
that involves dipping assemblies into gigantic baths of molten zinc, you can’t really re-galvanize
parts after the zinc has corroded to the point where it’s no longer protecting the steel. Also, in aggressive environments like the
coast or cold places that use deicing salts, a thin zinc coating might not last very long. In many cases, it makes more sense to use
an anode that can be removed and replaced, like I’ve done in my demonstration here. Cathodic protection anodes like this are used
on all kinds of infrastructure projects, especially those that are underground or underwater. I let this demonstration run for 3 weeks in
my garage. Each cycle lasted about 5 minutes, meaning
these samples were dipped in salt water just about 6,000 times. And here’s a timelapse of those entire three
weeks. Correct me if you find something better, but
I think this might be the highest quality time lapse video of corrosion that exists
on the internet. It’s actually really pretty, but if you’re
the owner of a bridge or pipeline that looks like this sample on the left, you’re going
to be feeling pretty nervous. You can see that the unprotected steel rusts
far faster than the other three and the rust attacks the sample much more deeply. The sample with the magnesium looks like it
was most protected from corrosion, but watch the anode. It’s nearly gone after just those three
weeks, and that makes sense. It’s the least noble metal on the galvanic
series by a long shot. The samples with aluminum and zinc anodes
do experience some surface corrosion, but it’s significantly less than the control. In fact, this is exactly how the lifespan
of the Howard Frankland bridge in Tampa was extended for so long. Zinc was applied around the outside of concrete
girders and in jackets around the foundation piles, then coupled to the reinforcing steel
within the concrete so it would act as a sacrificial anode, significantly slowing down the corrosion
of the vital structural components. Here’s a closeup of each sample after I
took them down from the Rustomatic 3000, and you can really see how dramatic the difference
is. The pockets of rust on the unprotected steel
are so thick compared to the minor surface corrosion experienced by the samples with
magnesium, aluminum, and zinc anodes. The anodes went through some pretty drastic
changes themselves. After scraping off the oxides, the zinc anode
is nearly intact, and you can even see some of the original text cast into the metal. The aluminum anode corroded pretty significantly,
but there is still a lot of metal left. On the other hand, there’s hardly anything
left of the magnesium anode after only three weeks. And here’s a look at the metal after I wire
brushed all the rust off each sample. The difference in roughness is hard to show
on camera, but it was very dramatic to the touch. There’s no question that the samples with
cathodic protection lost much less material to corrosion over the duration of the experiment. There’s actually one more trick to cathodic
protection used on infrastructure projects. Rather than rely on the natural difference
in potential between different materials, we can introduce our own electric current
to force electrons to flow in the correct direction and ensure that the vulnerable steel
acts as the cathode in the corrosion cell. This process is called impressed current cathodic
protection. In many places, pipelines are legally required
to be equipped with impressed current cathodic protection systems to reduce the chance of
leaks which can create huge environmental costs. The potential between the pipe and soil is
usually only a few volts, around that of a typical AA battery, but the current flow can
be in the tens or hundreds of amps. If you look along the right-of-way for a buried
pipeline, especially at road crossings, you can often see the equipment panels that hold
rectifiers and test stations for the underground cathodic protection system. The Howard Frankland bridge also had some
impressed current systems in addition to the passive protection to further extend its life,
proving a valuable lesson we learn over and over again: The maintenance and rehabilitation of existing
facilities is almost always less costly, uses fewer resources, and is less environmentally
disruptive than replacing them. You don’t need a civil engineer to tell
you that an ounce of prevention is worth a pound of cure (or the whatever the metric
equivalent of that is). It’s true for human health, and it’s true
for infrastructure. Making a structure last as long as possible
before it needs to be replaced isn’t just good stewardship of resources. It’s a way to keep the public safe and prevent
environmental disasters too. Corrosion is one of the number one ways that
infrastructure deteriorates over time, so cathodic protection systems are an essential
tool for keeping the constructed environment safe and sound. There’s one more video in this series on
corrosion, and (spoiler alert), it also includes a demonstration using the rustomatic 3000. If you want to catch that video as soon as
possible, you should know that my videos go live on Nebula before they’re released here
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